Trying to intersect two tunnels
There are several ways to do it, here is one:

Create your tunnel section, create an empty close to it, give your tunnel a Mirror modifier with the empty as Mirror Object, enable Axis, Bisect and Flip on the right axis (here X):




Rotate your empty 45° on the Z axis and move it so that the tunnel and its mirrored version cross each other:



Apply the modifier, go in Edit mode, select all, in the header menu &gt; Mesh &gt; Symmetrize, and in the Operator box choose the right axis:




Symmetrize in the other axis: